A sensor is essentially a device that responds to a physical, chemical, biological, or electrical stimulus by generating an electrical output signal that is related to the input electrical signal and a condition to be measured. There have been significant advances in the sensing industry based on requirements and applications in diverse fields such as biochemical sensing for security to curing of concrete. For example, the airport screening measures employ sensors for explosives, radioactive and/or biological dangers. Unfortunately, the present sensing technology lacks the sensitivity and ability to accurately detect a broad range of stimulus. This is particularly true with respect to employing the electronic sensors in environments which are unable to isolate the sensitive electronic components from the surrounding conditions.
An acoustic wave sensor responds to some input stimulus by the transconduction of the input stimulus to a perturbation of the electronic properties of the acoustic wave device. An output signal is produced by the acoustic wave sensing that represents a function of the input signal, the sensor element's response and the environmental input stimulus. In most cases the acoustic wave device is a passive circuit element, having an output that is at the same frequency as the input signal. Only the transfer efficiency and phase at a given excitation frequency typically change; however there are circuits that adjust the frequency of a signal to follow a phase or amplitude condition. It is often said that the frequency changes in such devices when, in fact, only velocity and length change at a physical level.
The acoustic wave sensor typically has a sensing area, such as a sensing film, wherein the device is sensitive to mechanical and electrical perturbations such as mass loading, viscoelastic property variations, electroacoustic interactions, flexure-frequency effects, and force-frequency effects. The sensor responds to the input stimulus with a corresponding change in the resonant frequency of the acoustic wave device or the phase shift and/or amplitude of the device at a specified frequency such that the change in frequency can be used to indicate properties of the stimulus. The use of acoustic wave sensors has led to many new applications and uses including in-field applications.
Current acoustic wave gas sensors are typically based on mass loading of a sensing film upon exposure to a target analyte. Mass loading refers to measuring changes of the vibrating member due to an increase of the mass caused by an adsorption of some gas. A mass loaded resonator has electrodes that convert energy between electrical and acoustic energy, wherein the device vibrates at a frequency determined by the input electrical energy signal. The degree and ease with which vibration occurs is determined by the proximity of the frequency of excitation to the device's resonant frequency or frequencies. In some cases the external circuit is devised so as to maintain the excitation at the resonant frequency as in an oscillator or phase locked loop. In others, a property of the signal is modified in a manner determined by the excitation frequency's proximity to said resonances. As the gas molecules are adsorbed by the sensing film, the added mass of the gas molecules causes a change in the propagation or resonance of the acoustic wave device. For such a device the resulting change is a frequency decrease in an oscillator or a phase shift increase at a fixed excitation frequency.
Some examples of the current type of sensors include capacitance-based sensors. These devices tend to use thick film polymers to form a sensor array. Another type of sensor is a SiC resonator which typically uses a pre-concentrator to increase sensitivity. The SiC resonator typically uses thick film polymers to construct a sensor array, such as 2-5 microns and uses mass loading for detection via induced shifts in the resonant frequency of the SiC plate.
Surface generated acoustic wave (SGAW) devices and Bulk Acoustic Wave (BAW) devices are used in many sensing applications, wherein a change in frequency of the sensor is related to the amount of mass that gets adsorbed onto or absorbed into the sensing film.
The limitation of current technology is that the sensing films in use have limited selectivity and customers demand proper identification of the analyte. Customers also seek multifunctional sensors that are capable of identifying and measuring multiple analytes in a mixture. This has led to the use of sensor arrays containing several discrete sensor elements, typically limited in the published art to four or eight sensors by manufacturing and design constraints.
Due to the inherent manufacturing variations and signal cross-talk issues, these arrays consist of sensors at different frequencies with unused guard bands between them. This results in unnecessarily tight manufacturing tolerances and limited array sizes in the cramped electromagnetic spectrum for wireless sensors and a generally difficult burden of multiplicity of design and complex spurious signal interactions for wired sensors. One general feature of the present invention is to incorporate an array of distinct sensing mechanisms into a single sensor.